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A B M P m e m b e r s ea r n F R E E C E h o u r s by rea d i n g t h i s i s s u e ! 33 inferior articular processes of the vertebra above. Z joints are the only synovial joints in the spine, with hyaline cartilage overlaying subchondral bone, a synovial membrane, and a joint capsule. Synovial f luid lubricates the joint, nourishes the articular cartilage, and facilitates smooth gliding of the articular surfaces. These joints function to stabilize the spine and limit axial rotation. Researchers estimate that Z joints carry about 25 percent of the dynamic axial load. 6 Together with the intervertebral discs, Z joints help distribute the weight of the upper body evenly along the vertebral column, preventing excessive stress on any single vertebra. DEGENERATIVE CHANGES IN Z JOINTS Like other synovial joints, Z joints can undergo degenerative changes over time. The breakdown of the articular cartilage, development of bone spurs (osteophytes), and a reduction in the joint's ability to absorb shock characterize osteoarthritis of the Z joints (Image 2). As osteophytes form, the bony outgrowths place pressure on adjacent structures such as spinal nerves, intervertebral discs, ligaments, the facet joint capsule, and nearby muscles. The synovium (the membrane that produces synovial f luid) often becomes inf lamed, exacerbating pain and stiffness. Associated conditions—like intervertebral disc degeneration—narrow the disc space, altering mechanical load distribution and resulting in progressive stress on Z joint articulations and joint capsules. 7 The cartilage and synovium of Z joints are sources of cytokines. Cytokines are potent mediators of inf lammation and can perpetuate cycles of inf lammation within the joint. Cytokines sensitize nociceptors, lowering the threshold for pain and making Z joints more sensitive to even minor stimuli. 8 The stimulation of sensory nerve fibers leads to the release of substance P and calcitonin gene-related peptide (CGRP). These neuropeptides cause vasodilation, increased blood f low, and further inf lammation. Persistent nociceptive signaling causes central sensitization in the spinal cord and brain, resulting in increased responsiveness of neurons in the central nervous system to threat signals, amplifying pain perception. 9 The musculoskeletal system responds to this inf lammatory cascade by sending muscles like the multifidus, rotatores, interspinales, intertransversarii, erector spinae, and quadratus lumborum into protective spasm. These muscles attempt to stabilize the vertebral column, causing additional pain, reduced movement, and inhibited muscle function. Additionally, inf lammatory cytokines can affect motor control pathways in the nervous system, leading to altered muscle recruitment patterns and loss of coordination. 10 POSTURAL CAUSES OF OSTEOARTHRITIS IN Z JOINTS Injury, trauma, obesity, genetic predisposition, and age contribute to the development of Z joint osteoarthritis. Poor In chronic cases, diffuse low-back pain may spread into the buttocks, groin, or down the leg. A B M P m e m b e r s ea r n F R E E C E h o u r s by rea d i n g t h i s i s s u e ! 33 2 posture is a critical factor for much of the population experiencing low-back pain. Throughout adult life, the superior and inferior facets of the lumbar vertebrae ref lect the different stresses placed upon them. As you might suspect, damage occurs in regions that experience the greatest mechanical forces. 11 Often, the cartilage in the central region of the facet joints is spared, with the majority of changes occurring where the articular processes contact one another during f lexion and extension. 12 Any postural habits that disrupt the spine's natural curvature exacerbate this type of wear on Z joint cartilage. Examples include excessive lordosis caused by locked knees in standing postures or excessive kyphosis caused by slouching over a keyboard. Poor posture involves muscle imbalances that can alter the movement patterns of the spine, affecting the alignment and function of the Z joints. 13 Similarly, poor posture in weight- bearing positions increases joint compression, accelerating cartilage degeneration. Degeneration is worsened by repetitive movement and work activities involving frequent heavy